A major problem and inconvenience in die casting equipment is maintaining a working fit between the cold chamber and shot tip or plunger which pressures the molten metal into the die. Continued cycling of the shot tip in use results in wear of the shot tip which enables molten metal to enter the space between the shot tip and cold chamber. As the tip wears, the metal easily enters this area because of the high pressure to which the molten metal is subjected during the injection process. The most common method of maintaining the fit of the shot tip in the shot sleeve is to interrupt operation of the die casting machine and manually hammer the shot tip to expand the tip. This technique is very crude and can severely damage the tip and does not provide for uniform expansion of the full diameter of the shot tip, and thus the hammered and expanded tip does not maintain the desired sealing fit for more than a few cycles of the machine.
The Bauer Patent No. 2,932,865 discloses several arrangements to maintain the desired working fit of a shot plunger with the cold chamber. One technique disclosed in Bauer is to use a separate throw-away plunger which operates in advance of the shot tip. In other arrangements, the shot tip itself is provided with a special expandable rim which is deformed under pressure when in use. The techniques disclosed in Bauer have not been recognized in the trade as a satisfactory solution to the problem.
The German Patent No. 742,182 appears also to disclose a shot tip or plunger with an expandable rim, and thus is in the same category as Bauer.